Publication: Ultrastructure of invertebrate muscle cell types
Authors
Paniagua, R. ; Royuela, M. ; García-Anchuelo, R.M. ; Fraile, Benito
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Publisher
Murcia : F. Hernández
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DOI
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info:eu-repo/semantics/article
Description
Abstract
The muscular cells of invertebrates can be
divided into three major classes on the basis of their
striation pattem: transversely striated, obliquely striated,
or smooth muscle. Transversely striated muscles have
either continuous or discontinuous Z lines and, thus, can
be subdivided into two types respectively. Of al1
invertebrate muscles, the transversely striated muscle
with continuos Z lines is the most similar to the
vertebrate skeletal muscle and is present in arthropods,
whose musculature (including the visceral muscles) only
consists of this cell type. These muscles are multinucleate
cells that contain myofibrils showing welldefined
sarcomeres. Transversely striated muscles with
discontinuous Z lines, consisting of multiple small
electrondense patches, are found in the translucent
portions of adductor muscles of some bivalves and in the
heart muscle of the gastropods. This muscle is formed by
mononucleated cells with centrally-located nuclei and a
single myofibril. The obliquely striated muscle appears
in nematodes, annelids, molluscs, brachiopods and
chaetognathes and consists of mononucleated cells with
both thick and thin myofilaments which form
sarcomeres delimited by Z lines. Myofilaments are not
perpendicular but oblique to the Z lines, so that both A
and 1 bands may be seen together in each of the three
spatial planes of view. Smooth muscle has been reported
in coelenterates, annelids, molluscs, brachiopods and
echinoderms, but is lacking in arthropods. These muscle
cells have a centrally-located nucleus and abundant thin
and thick myofilaments without apparent sarcomeres.
The most relevant characteristics of invertebrate muscle
cells are the following. The thick (myosin) myofilarnents
show a variable length (from 2.2 pm up to 6 pm) and width (from 14 nm up to 231 nm) and contain a central
core of paramyosin, which is absent in vertebrate
muscles. Thick filaments are homogenous in
transversely striated muscles and either homogeneous or
fusiform in the obliquely striated and smooth muscles.
Thin filaments measure 6 nm in diameter. They contain
tropomyosin and, only in striated muscles, also troponin. The thinlthick filament ratio varies from 311 to 611, even
in smooth muscles. The plaques for filament anchorage
(Z lines in striated muscles or electrondense bodies in
smooth muscles) contain a-actinin. The striated
(transversely or obliquely) muscles show long
sarcomeres (up to 9 pm) and the number of thin
filaments around each thick filament varies from 3 to 12,
so that each thin filament is shared by two thick
filaments. Z lines in the striated muscles show a variety
of structures that differ from one species to another
(filament bundles in nematodes, bars in annelids, small
patches in molluscs, etc). Many striated muscles contain
titin (connectin) and intermediate filaments and display a
sarcotubular system consisting of T tubules and
sarcoplasmic reticulum tubules. Both structures form
dyads and, more rarely, triads. The location of T tubules
as well as the configuration and distribution of
sarcoplasmic reticulum vary among muscles and species.
Invertebrate smooth muscle differs from that of
vertebrates principally in the higher proportion and
larger diameter of thick myofilaments. These may be
fusiform and their size and number may vary widely
among cells. These muscle cells may be classified by the
characteristics of both the thick filaments and the
electrondense bodies for filament anchorage.
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